There is conventionally known an image reading apparatus which is mounted on a copy machine, a scanner, and a multi function device that includes the functions of the copy machine and the scanner. Such an image reading apparatus includes a flatbed scanner and an apparatus that includes an automatic document conveying mechanism called an ADF (Auto Document Feeder) in which a CCD (Charge Coupled Device) or a CIS (Contact Image Sensor) is disposed and which conveys a document from a paper supply tray to a paper discharge tray through a conveying path, and reads an image of the document to be conveyed.
For example, in a conventional image reading apparatus 90 shown in FIG. 16, a document pressing cover 91 is provided with an ADF 92. A document of a paper supply tray is conveyed to an ADF reading region 93A of a platen glass 93 by the ADF 92. Then, by an image scanner (not shown) disposed in a document placing table 94, the image of the document traveling through the ADF reading region 93A can be read. Further, the image reading apparatus 90 can be used as a flatbed scanner. In this case, the document placed on a still document reading region 93B of the platen glass 93 is covered with the document pressing cover 91 to be then pressed and fixed. Then, the image of the document can be read by the image scanner disposed in the document placing table 94.
In a CCD or CIS used as an image sensor, light is emitted from a light source toward the platen glass 93, transmits the platen glass 93, and is reflected from the surface of the document. And then, reflected light is converted into an electrical signal by a light-receiving element, and thus the image of the document is optically read. The light source or the light-receiving element of the image sensor has a variation in light intensity, light distribution irregularity, or sensitivity irregularity. For this reason, so-called shading correction is performed. In the shading correction, after the light intensity of the light source for a white reference member is adjusted, the reference member is read so as to acquire white level data and black level data, and reading data is corrected on the basis of white level data and black level data. For example, in the image reading apparatus 90, a reference member is disposed at the back of a dividing member 95 which divides the platen glass 93 into the ADF reading region 93A and the still document reading region 93B. According to this configuration, prior to image reading of the document, the light intensity adjustment about the reference member is performed and reference data is acquired.
In recent years, with the enhancement of sensitivity of the image sensor, a light source having low illuminance becomes usable. For example, in the CIS, an LED (Light Emitting Diode) light source or an LIDE light source using an LED and a linear light guide is adopted. Due to low illuminance of the light source, there is a condition in that an influence of external light on the reference member is increased. External light is indoor or outdoor light entering from the ADF reading region 93A or the like into the document placing table 94 when image reading by the image scanner is performed in a state in which, in the image reading apparatus 90, the document is placed on the still document reading region 93B of the platen glass 93 and the document pressing cover 91 is closed, as shown in the drawing.
If the document pressing cover 91 is closed, the entire region of platen glass 93 is covered with the document pressing cover 91, and thus external light doe not enter into the document placing table 94. However, at the time of image reading of a document such as a thick book or the like, the document pressing cover 91 cannot be completely closed, and thus external light may enter into the document placing table 94. Further, at the time of image reading of a document having a large size, in order to confirm a reading position, image reading may be performed with the document pressing cover 91 unclosed. In such a case, external light enters into the document placing table 94.
When illuminance of the light source is larger than external light, reflected light at the reference member is sufficiently increased. Therefore, even when some external light is added to reflected light, there is no case in which the read image is drastically degraded. However, if illuminance of the light source is small, reflected light is decreased, the ratio of external light to be added to reflected light becomes large. For example, if external light has influence on the light intensity adjustment, the light source of the image sensor is adjusted to have a smaller light intensity by the amount of external light. Accordingly, there is a condition in that the amount of light to be irradiated onto the document from the light source is decreased, the read image is darkened. In particular, if the image sensor scans the document placed in the still document reading region 93B, as the image sensor is distant from the ADF reading region 93A, the influence of external light is decreased, and thus image degradation drastically appears. In order to solve this situation, for example, there is suggested a unit which prevents external light from entering into the light-receiving element of the image sensor by light-shielding plates provided on both sides of the image sensor (see JP-A-2003-134307).